This manuscript presents the microstructure, and mechanical analysis of 3 mm thick Al 6101 (aluminum alloy) and C11000 (Cu alloy) plates Friction Stir Welded joints with and without Silver and Zinc interlayers. The tool used to make joints is made of H 13 steel, having a shoulder with 18 mm diameter and a taper pin with 4 mm tip and 6 mm root diameter. The quality of welded joints has been inspected using tensile testing, micro-hardness tests, scanning electron microscopy, and energy dispersive X-ray analysis. Microstructural studies clarified the flow variation of constituents with and without interlayer. Incorporation of a Silver and Zinc interlayer between the joining surface of aluminum and copper alloy decreased the generation of brittle intermetallics. The joint made with hybrid interlayers (Silver and Zinc) has the highest tensile strength with complete ductile fracture compared to the other joints tested.
Research Article
Microstructural and Mechanical Characteristics of Friction Stir Welded Al6101/C11000 Joints with Zinc and Silver Interlayer
https://doi.org/10.21203/rs.3.rs-2215956/v1
This work is licensed under a CC BY 4.0 License
Journal Publication
published 26 Jul, 2023
Read the published version in The International Journal of Advanced Manufacturing Technology →
You are reading this latest preprint version
Friction Stir Welding
Aluminium alloy
Copper alloy
Intermetallics and Microstructures
The Welding Institute (TWI) first demonstrated the solid-state joining technique known as friction stir welding (FSW) in 1991. Since then, the technique's popularity among researchers has steadily increased. The fabrication sector is increasingly adopting this technique as a result of many benefits that it offers in comparison to fusion welding. Typically, FSW plasticizes the material by generating low heat input in the weld zone. Hence without melting or solidification, the resultant joints form [1].
In FSW, a non-consumable rotating tool is inserted between the sheets' abutting edges and driven toward the joint line. This tool is constructed with specific geometry of the shoulder and probe to regulate material movement and flow pattern. Heat is generated due to friction between the tool and the workpiece. This heat softens the material that is adjacent to the tool and results in an excessive amount of plastic deformation. The tool rotation stirs the softened material and pushes behind the tool by the probe [2]. Because of the combined action of frictional heat and excessive deformation of the workpiece by tool, the forging and stirring that took place resulted in a high-quality metallurgical bond [3–5].
Copper and its alloys have favorable electrical, thermal, corrosion, and fatigue properties and are simple to produce. Despite its excellent electrical conductivity, copper is not suitable for lightweight applications. Furthermore, due to its relatively high price, it incurs considerable costs. As a result, aluminum, a less dense and less expensive metal with similar electrical properties, has been substituted. But copper can't be replaced entirely in every application, so the most practical approach is to create their joints. On the one hand, the copper and aluminum joints reduce the structure's weight and cost while also utilizing copper's inherent properties [6]. The creation of Al-Cu-based joints is highly challenging because of their differing chemical, mechanical, and thermal characteristics and also generate brittle intermetallic compounds (IMCs). Due to the need to reduce weight, improved performance, and economical aspect, makes the effective combination of radically dissimilar metals like Al and Cu move exciting in the field of chemical, nuclear, electrical, aviation, automotive, and electronics industries.
Researchers' curiosity increased in investigating the microstructure and mechanical properties of FSWed Al–Cu joints in the last few decades [7]. The growth and breakage of the IMCs, which diminishes joint strength, is the most common cause of failure to achieve joints with desirable characteristics. When Al and Cu alloys are directly joined, the hard and brittle IMCs are easily produced due to their high chemical affinity. However, by putting the interlayer in the center of the Al/Cu junction, the generation of aluminum-copper IMCs can be reduced, and the joint's mechanical characteristics can be enhanced [8]. Researchers have recommended using interlayers for preparing sound joints [9–11].
The addition of interlayers like Zn and Ag has resulted in the formation of intermetallics like\({\text{A}\text{g}}_{2}\text{A}\text{l}\),\(\text{A}\text{l}\text{Z}\text{n},\) \({\text{C}\text{u}\text{Z}\text{n}}_{5}\) which is less detrimental than\({ \text{A}\text{l}}_{4}{\text{C}\text{u}}_{9},\) \({ \text{A}\text{l}}_{3}{\text{C}\text{u}}_{4},\) \({\text{A}\text{l}}_{2}\text{C}\text{u}\), \(\text{A}\text{l}\text{C}\text{u}\)IMCs formed for FSWed joints without interlayers. The literature review has revealed that the studies on FSWed Al-Cu joints with interlayer are much less on Al6101/C11000, which is widely used for busbar application. It produces a scope to explore the role of interlayer in enhancing the welding characteristics of FSWed Al-Cu joints. Hence it is proposed to study the influence of interlayers like Zn and Ag individually and in hybrid on Al6101/C11000 joints and their impact on microstructural features and mechanical properties.
2.1 Base Plate
Aluminum alloy 6101 (AA 6101) and Copper C11000 with dimensions of 150mm×60mm×3mm was used as base plate. Tables 1 and 2 show the chemical composition and mechanical properties, respectively.
|
Mg |
Si |
Al |
Cu |
|
|---|---|---|---|---|
|
AA 6101 |
0.6 |
0.5 |
98.9 |
|
|
C11000 |
99.9 |
|
Materials |
Density |
Tensile Strength |
Elastic modulus |
Melting Point |
|---|---|---|---|---|
|
AA 6101 |
2.7 (g/cm3) |
97 MPa |
70–80 GPa |
588°C |
|
C11000 |
8.89 (g/cm3) |
220 MPa |
117 GPa |
1083°C |
Before welding, the plates were machined to ensure that the mating surfaces were perfectly smooth and aligned. Filing was used to eliminate milling-induced burrs and to improve plate-to-plate contact. Finally, the plates surface were polished with emery to remove oxide layer if any, followed by acetone washing to remove foreign particles if any.
Figure 1 depicts the arrangement used in this study for FSW. The joint \({\text{J}}_{1}\) is constructed without an interlayer, as shown in Fig. 1(a). Whereas joints \({\text{J}}_{2}\) and \({\text{J}}_{3}\) (Fig. 1 (b & c)) are created with the incorporation of silver and zinc interlayer, respectively, and joint \({\text{J}}_{4}\) (Fig. 1(d)) is produced with the incorporation of both silver and zinc as a hybrid interlayer. The thickness of each interlayer is 0.2 mm with a 150 mm length and a 3 mm breadth.
2.2 Tool
The geometry of tool used for FSW in this research is shown in Table 3.
|
Tool material |
H-13 tool steel |
|---|---|
|
Pin shape |
Taper threaded |
|
Pin geometry |
Root diameter = 6 mm Tip diameter = 4 mm Pin length = 2.7 mm |
|
Shoulder shape |
Outer surface = Cylindrical End surface feature = 3° concavity from the flat shoulder end surface, Scroll surface |
|
Shoulder geometry |
18 mm diameter 15 mm length |
|
Shank geometry |
20 mm diameter 67.1 mm length |
A taper threaded pin tool profile was used in the experiments (Fig. 2). For the tool's tapered pin, its root was 6 mm, and tip was 4 mm in diameter, and its height was 2.7 mm. The most common FSW material is tool steel due to its low cost, wide availability, machinability, and widespread acceptance. H-13 is a hot-worked, air-hardened chromium-molybdenum steel. It has heat, wear, and fatigue-resistant and durable [12]. The shoulder end surface has a 3° concavity with the scroll surface. Extraction of material from the shoulder side can be prevented by providing concavity on the end surface of the shoulder [13–15].
During plunging, material displaced by the probe is fed into the tool shoulder cavity. The tool's concave shoulder serves as a reservoir for probe material. The material within the concave shoulder displaced and forged the material behind the tool during the tool's pressing. Moving the tool forward will pull material from behind the probe and force it into the shoulder cavity. Combining the concave shoulder and the scrolling design helps to minimize tool lift when welding occurs at fast speeds. By entrapping plasticized material in special reentrant features, the scrolling shoulder fixes the undercut fault generated by the concave tool and decreases flash. Shearing the material in channels (reentrant features) on the surface of the workpiece results in deformation and frictional heating. Taper tools generate higher frictional heat because of the greater contact area between the pin and the workpiece. As a result, it offers more plastic deformation. The tapered probe's capacity to provide substantial hydrostatic pressure in the weld zone improves material stirring and nugget integrity. Material can be circulated around the tool several times with a threaded pin before being placed behind the tool. Thus, the threaded pin facilitated the stirring of material, filling the voids and breaking formed oxides [16–19].
2.3 Processing and characterization of Al6101-Cu (C11000) plates
FSW was done using the parameters specified in Table 4 by keeping Cu in advancing and Al in retreating side. A 3-ton friction stir welding machine (WS005, ETA Technology, Bangalore) was utilized for the FSW process. To analyze the role of the interlayer individually and in hybrid mode on the mechanical behavior of the FSWed Al-Cu plates, samples were examined under a field emission scanning electron microscope (FESEM sigma 300), and their microstructure was characterized. FSWed samples were subjected to mechanical testing to analyze the tensile property as per standard (ASTM-E8), and fractography was also done on the tested sample to characterize its mechanical behavior.
Table 4: Parameters of FSWed joints
3.1 SEM analysis of joint cross-section
With the aid of a scanning electron microscope (SEM) equipped with an energy dispersion spectroscope (EDX), the distribution of individual elements and the composition of IMCs have been explored across the nugget zone/stir zone. The four joints formed (joint\({\text{J}}_{1},\) \({\text{J}}_{2}\),\({ \text{J}}_{3,} { \text{J}}_{4}\)) have all been analyzed. The interlayer was placed at the interfacing surface of Al-Cu. Figure 3 depicts the SEM micrograph of the FSWed region of all produced joints: (a) FSWed region of Al-Cu (b) FSWed region of Al-Cu with Ag interlayer (c) FSWed region of Al-Cu with Zn interlayer (d) FSWed region of Al-Cu with hybrid Ag and Zn interlayer, with silver placed on the aluminum side and zinc on the copper side.
The joint\({\text{J}}_{1}\)'s NZ demonstrates that the mixture of Al6101 and C11000 has resulted in uniform dispersion and indicates the presence of IMCs. The fluctuation in the atomic percentage (at. %) of these Al and Cu alloys at different spots demonstrates the presence of different IMCs. Figure 4 depicts the EDX-examined NZ spots of joint\({ \text{J}}_{1}\). Figures 5 & 6 display the examined at. % of Al and Cu alloys at specific locations. Table 5 represents the possible phases for these alloys, which are in line with the research studies reported by researchers [20–23]. Researchers have spotted Cu-rich phases relatively close to the Cu side.
The varying at. % of Al-Cu alloys in NZ support the presence of several intermetallics. The affinity and interaction of Al-Cu has developed into intermetallics with reference to their phase diagram [5, 24].
|
Points |
Al (at. %) |
Cu (at. %) |
Possible phase |
|---|---|---|---|
|
1 |
67.15 |
32.85 |
\({\text{A}\text{l}}_{2}\text{C}\text{u}\) |
|
2 |
31.23 |
68.77 |
\({\text{A}\text{l}}_{4}{\text{C}\text{u}}_{9}\) |
Additional evidence of the distribution of individual metal particles in the nugget zone is provided by the elemental mapping (Fig. 7). Line scan of the NZ was carried out to comprehend element distribution and related intermetallics better. The elemental mapping of the FSWed joint \({\text{J}}_{1}\)is given in Fig. 7a and 7b. These figures use red and green colours to show the dispersion of Al and Cu alloy particles, respectively. According to the distribution pattern, copper particles are spread uniformly throughout the aluminium matrix. Consequently, the NZ resembles like aluminium matrix composite structure.
Figure 7c depicts the NZ's exact viewpoint for the joint\({\text{J}}_{1}\). The element's line scan and its individual element's intensity are shown in Fig. 7c, showing that Al has a higher intensity than Cu because the pin offset is towards Al.
EDX was used to examine the particular spot marked in NZ and near the Al side in Figs. 8a and 8b for joint\({ \text{J}}_{2}\), and the findings are presented in Figs. 9 and 10. Table 6 shows the at. % of each element (Al, Ag, and Cu) present at different spots in Fig. 8. It is evident that IMCs have formed from the fluctuation of at. % Al-Cu with Ag interlayer particles. IMC such as \({\text{A}\text{g}}_{3}\text{A}\text{l}\)or \({\text{A}\text{g}}_{2}\text{A}\text{l}\) is likely to be created via Al/Cu/Ag binary and ternary phase diagrams [25, 26].
|
Points |
Al (at. %) |
Ag (at. %) |
Cu (at. %) |
Possible phase |
|---|---|---|---|---|
|
1 |
27.8 |
70.7 |
1.5 |
\({\text{A}\text{g}}_{3}\text{A}\text{l}\)or \({\text{A}\text{g}}_{2}\text{A}\text{l}\) |
|
2 |
97.9 |
1.93 |
0.17 |
Al |
The line scan and elemental mapping of the joint\({ \text{J}}_{2}\)'s weld interface are shown in Fig. 11 (a) to (c). Al and Cu alloys are separated along the Ag interlayer's (sky colour) surface. This interlayer can prevent several intermetallic formations. The distribution of each constituent is shown in Fig. 11b. Line scans have shown that the intermetallic development in this instance is minimal, homogeneous, and thinner than the joint\({ \text{J}}_{1}\). The formation of thin and uniform intermetallic with third material as interlayer is beneficial in dissimilar joints [5]. Thin, uniform continuous intermetallic layer gives sound joint in FSW of Al-Cu [27].
Figure 12 depicts the SEM examination results of the NZ of the joint\({\text{J}}_{3}\), and Fig. 13 and 14 show the at. % of each component (Al, Zn, Cu) at various locations examined by EDX. It was also revealed that many binary and ternary compounds are formed under these conditions. Figure 13 and 14 show that EDX-1 was made up of\({\text{A}\text{l}}_{4.2}{\text{C}\text{u}}_{3.2}{\text{Z}\text{n}}_{0.7}\), while EDX-2 was found to be made up of\(\text{C}\text{u}{\text{Z}\text{n}}_{5}\). These compounds were formed due to the diffusion of Zn with Al-Cu alloys in the NZ during FSW [8, 28, 29]. Table 7 shows the at. % of each element (Al, Zn, and Cu) present at different spots in Fig. 12.
|
Points |
Al (at. %) |
Zn (at. %) |
Cu (at. %) |
Possible phase |
|---|---|---|---|---|
|
1 |
52.01 |
11.78 |
36.21 |
\({\text{A}\text{l}}_{4.2}{\text{C}\text{u}}_{3.2}{\text{Z}\text{n}}_{0.7}\) |
|
2 |
1.08 |
83.72 |
15.2 |
\(\text{C}\text{u}{\text{Z}\text{n}}_{5}\) |
The line scan and elemental mapping of the joint\({ \text{J}}_{3}\)'s weld interface are shown in Fig. 15 (a) to (c). Al and Cu alloys are separated along the Zn interlayer's (yellow colour) surface. The uniform diffusion of the Zn interlayer across the joint interface can prevent several intermetallic formations. The distribution of each constituent is shown in Fig. 15b. The line scan along the interface of joint\({ \text{J}}_{3}\) in Fig. 15c shows the diffusion of Cu and Zn in the Al matrix. The distribution of Cu and Zn in Al formed the \({\text{A}\text{l}}_{4.2}{\text{C}\text{u}}_{3.2}{\text{Z}\text{n}}_{0.7}\) phase, and the diffusion of Cu and Zn created the \(\text{C}\text{u}{\text{Z}\text{n}}_{5}\) phase.
Figure 16 illustrates the outcomes of an SEM analysis of the \({\text{J}}_{4}\) joint's NZ. The at. % of each metal (Al, Ag, Zn, and Cu) at various locations examined by EDX is shown in Figs. 17 and 18. As indicated in Figs. 17 and 18, EDX-1 was determined to be\(\text{C}\text{u}{\text{Z}\text{n}}_{5}\), but EDX-2 was found to be \({\text{A}\text{g}}_{2}\text{A}\text{l}\) IMCs. These IMCs seem less detrimental than intermetallics like \(\text{A}\text{l}\text{C}\text{u},\) \({\text{A}\text{l}}_{2}\text{C}\text{u},\) \({\text{A}\text{l}}_{2}{\text{C}\text{u}}_{3},\) \({\text{A}\text{l}}_{4}{\text{C}\text{u}}_{9}\), etc. Hence, it is anticipated that the mechanical properties of such welding may be significantly better compared with others. Table 8 shows the at. % of each element (Al, Ag, Zn, and Cu) found at different spots in Fig. 16. It was also expected to have some softness and ductility in the nugget zone.
|
Points |
Al (at. %) |
Ag (at. %) |
Zn (at. %) |
Cu (at. %) |
Possible phase |
|---|---|---|---|---|---|
|
1 |
0.22 |
0.86 |
81.72 |
17.2 |
\(\text{C}\text{u}{\text{Z}\text{n}}_{5}\) |
|
2 |
34.37 |
65.38 |
0.13 |
0.12 |
\({\text{A}\text{g}}_{2}\text{A}\text{l}\) |
The line scan and elemental mapping of the joint\({ \text{J}}_{4}\)'s weld interface are shown in Fig. 19 (a) to (c). Al and Cu alloys are separated along the Ag (mainly) and Zn interlayer's (sky and yellow colour, respectively) surface. Compared to Zn, Ag works as a diffusion layer because it has a higher melting point. Ag and Zn interlayers prevent hazards intermetallic by blocking Al-Cu direct contact. The diffusion of Zn across the nugget zone is uniform, and a thin layer of Zn can be seen on the Cu side. The distribution of each constituent is shown in Fig. 19b. The line scan of joint\({ \text{J}}_{4}\) along the weld interface is given in Fig. 19c, where silver intensity is higher than the other elements. Cu and Zn particles are evenly dispersed throughout the Al matrix of the NZ. The diffusion of Cu-Zn and Al-Ag formed the\(\text{C}\text{u}{\text{Z}\text{n}}_{5}\)and \({\text{A}\text{g}}_{2}\text{A}\text{l}\) phases. The line scan reveals the formation of the Al-Ag-Zn-Cu diffusion phase also. The reactivity of Al-Ag and Cu-Zn to form the\({\text{A}\text{g}}_{2}\text{A}\text{l}\) and \(\text{C}\text{u}{\text{Z}\text{n}}_{5}\) phases is strong. Hence, the Ag interlayer is placed towards the Al side and Zn towards the Cu side.
The weld nugget morphologies of the weld conducted with and without interlayer have been shown through elemental mapping in Fig. 20(a-d). Aluminium is represented by the colour red, copper by the colour green, silver by the colour blue, and zinc by the colour yellow. The distribution of the copper bulk and some copper particles is seen in Fig. 20a, where it has mixed with the Al base metal. Since the pin offset was toward Al, it served as a matrix material in which copper particles get embedded. Figure 20b displays the Cu-mixed lamellae and the uniform mixing of Ag particles. In the nugget zone of joint\({ \text{J}}_{3}\), entrapped Zn particles in the lamellae of aluminum and copper are visible (Fig. 20c). The distribution of smaller particles and a more significant proportion of Cu-rich precipitates can also be observed. Figure 20d shows the aluminium matrix's intercalated Cu, Ag, and Zn layers. These appear to be wave-like layers, similar to what Muhammad et al. observed [30].
3.2 Variation of Tensile Strength
This section presents the results of tensile testing performed on all FSWed joints. Figure 21 shows that all welded specimens failed almost in the stir zone. Transverse tensile tests show that when interlayer is used, the tensile strength of dissimilar welds increases. Figure 22 depicts the ultimate tensile strength (UTS) of joints\({ \text{J}}_{1,}{ \text{J}}_{2,}{ \text{J}}_{3}\)and\({\text{J}}_{4}\). UTS values for FSWed joints\({ \text{J}}_{1,}{ \text{J}}_{2,}{ \text{J}}_{3}\)and\({\text{J}}_{4}\) are 89.26, 113.47, 99.84, and 121.78 MPa, which correspond to 92, 117, 103, and 125% of the Al base metal, respectively.
Here, the reason behind the difference in tensile properties of FSWed joints (without interlayer, with Ag, with Zn, and hybrid Ag-Zn interlayer) is discussed. The joint \({\text{J}}_{1}\) produced without incorporating the interlayer, i.e., the FSW of Al6101 and C11000. The dispersion of copper in Al can be noticed in Fig. 7a & 7b. The mixing of Al-Cu resulted in FSWed joint\({\text{J}}_{1}\), but the distribution of Cu particles in Al produced hard and brittle IMCs like \({\text{A}\text{l}}_{2}\text{C}\text{u},\) and \({\text{A}\text{l}}_{4}{\text{C}\text{u}}_{9}\) as noticed in EDX results Figs. 5 & 6. The same has been observed by other researchers [31–33]. Hence, these hazardous IMCs reduce the strength of the joints. The formation of harmful intermetallics of Al-Cu can be decreased by incorporating the interlayer at the faying surface of the parent metals [34].
Hence, for producing FSWed joint\({ \text{J}}_{2}\), silver (Ag) is incorporated as an interlayer between the Al6101-C11000 FSW butt configuration. The ultimate tensile strength result of joint \({ \text{J}}_{2}\) has been improved and it is better than the case with no interlayer (\({ \text{J}}_{1}\)) because the silver (Ag) interlayer hinders the direct contact between Al-Cu. Within the stir zone, lustrous particles are detected (Fig. 11b), scattered within the Al matrix, and it maybe silver (Ag) or compounds of Ag, Al, and Cu. The EDX result reveals the formation of intermetallic \({\text{A}\text{g}}_{2}\text{A}\text{l}\) of the Al-Ag system. This intermetallic \({\text{A}\text{g}}_{2}\text{A}\text{l}\) is mixed with Al, Cu, and Ag layers, combined with the Al matrix, creating a composite type structure. The tensile strength of the FSWed joint \({\text{J}}_{2}\) with an Ag interlayer got better because the brittle intermetallic \({\text{A}\text{l}}_{4}{\text{C}\text{u}}_{9}\) didn't grow as much, and a more ductile \({\text{A}\text{g}}_{2}\text{A}\text{l}\) formed instead. Intercalated \({\text{A}\text{g}}_{2}\text{A}\text{l}\) intermetallics acted as reinforcements, and intercalated with layers of work material AA6101 and C11000, along with dispersed Ag particles, and strengthenes the weld. Shailesh et al. also found similar results, like how the development of \({\text{A}\text{g}}_{2}\text{A}\text{l}\) intermetallic prevented \({\text{A}\text{l}}_{4}{\text{C}\text{u}}_{9}\) formation and increased the tensile strength of dissimilar joints (AA6082-O to pure copper) when silver (Ag) was used as an interlayer [10].
Figure 15 (a & b) shows that Zn particles are mixed with base metal in the FSWed joint\({ \text{J}}_{3}\). A composite-like structure comprising many base metal particles stuck together in the stir zone's centre. When Zn was used as an interlayer, it stopped AA6101 and C11000 from making a thick intermetallic layer. As a result of the low melting point of zinc (Zn) compared to aluminium and the influence of the tool's stirring, the interlayer was spread fairly throughout the NZ. This led to thin, uniform IMCs in the nugget zone, strengthening the weld [29]. It was found that joint \({\text{J}}_{3}\) (with Zn interlayer) has a higher tensile strength than joint \({\text{J}}_{1}\) (without interlayer) but a lower tensile strength than joint \({\text{J}}_{2}\) (joint with Ag interlayer). The uniform distribution of Zn at the nugget zone prevented the parent metals from interacting directly, hence avoiding the growth of potentially hazardous Al-Cu intermetallics. When Zn reacts with copper, it forms an intermetallic called\(\text{C}\text{u}{\text{Z}\text{n}}_{5}\) [8]. Even though \(\text{C}\text{u}{\text{Z}\text{n}}_{5}\) is brittle IMCs, the formation of composite-like structures by the intercalation of Zn with Al-Cu and the distribution of uniform and thin intermetallic at the nugget zone gave more tensile strength than joint \({\text{J}}_{1}\) but less than joint \({\text{J}}_{2}\) because the intermetallic \({\text{A}\text{g}}_{2}\text{A}\text{l}\) in joint \({\text{J}}_{2}\) is more ductile than \(\text{C}\text{u}{\text{Z}\text{n}}_{5}\).
Joint\({\text{J}}_{4}\), out of all the FSWed joints tested, was found to have the highest tensile strength due to several intercalated compounds of Ag and Zn with Al-Cu in the nugget zone (Fig. 20d) and the uniform distribution of Zn-generated thin intermetallic and Ag-generated ductile \({\text{A}\text{g}}_{2}\text{A}\text{l}\) intermetallic. The composite-like structures\({\text{A}\text{g}}_{2}\text{A}\text{l}\),\(\text{C}\text{u}{\text{Z}\text{n}}_{5},\) and potential intercalated compounds of Ag and Zn with Al-Cu in the nugget zone achieved the best tensile strength with the hybrid interlayer.
Joints \({\text{J}}_{2}\)and\({ \text{J}}_{3}\)have good strength compared to the UTS without interlayer\({ \text{J}}_{1}\), however the hybrid interlayer joint \({\text{J}}_{4}\) has the maximum. The creation of potentially hazardous brittle Al-Cu intermetallic is inhibited by the fine and uniformly distributed Ag and Zn interlayers, which produce ductile \({\text{A}\text{g}}_{2}\text{A}\text{l}\) intermetallic and many composite-like structures in the nugget zone. With proper diffusion, solid solution strengthening, and thin, uniform, and controlled development of IMCs, the interlayer strengthens the joints to a greater extent than the Al base metal's tensile strength. The fine grain size of the Al matrix and the spread of the fine IMC particles contribute significantly to the high strength of the composite structure [5, 27, 35, 36].
3.3 Fractography
The FESEM pictures were taken to analyze the fracture surface morphologies to identify the causes of the four joint failures. Each joint's failure mode is interrogated, considering the fracture surface. Images of the fracture surfaces of the nugget zone-failing joints \({\text{J}}_{1},\) \({\text{J}}_{2},\) \({\text{J}}_{3},\) and \({\text{J}}_{4}\) are shown in Fig. 23 (a-d). Fracture surfaces of Al-Cu FSWed joint \({\text{J}}_{1}\) is demonstrated in Fig. 23(a). As a point of interest, the fracture surface showed no dimples—poor weld strength results from broken grains having an irregular shape rather than a uniform dimple shape. As a result, brittleness was the mode of fracture for this specimen. Similar conclusion was achieved by researchers in other Al-Cu systems [37]. The SEM picture in Fig. 23(b) shows how the fracture surfaces of the Al-Cu FSWed joint \({\text{J}}_{2}\) evolve after the incorporation of the Ag interlayer. The fracture surface is a mix of equiaxed dimples and a few flat spots. The image shows many homogeneous dimples, indicating that aluminium and copper form a strong metallic bond. The dimples' extension in the loading direction depicts the ductile failure, while the fracture surface's flatness shows the brittle failure [20, 38]. The SEM picture in Fig. 23(c) shows how the fracture surfaces of the Al-Cu FSWed joint \({\text{J}}_{3}\) evolve after the incorporation of the Zn interlayer. Shallow dimples can be observed on the fracture surface. This type of morphology indicates that the fracture behaviour is more brittle than ductile [39–41]. \(\text{C}\text{u}{\text{Z}\text{n}}_{5}\), an intermetallic formed when Zn interacts with copper, is brittle but less harmful than other Al-Cu intermetallics such as \({\text{A}\text{l}}_{2}\text{C}\text{u},\) and\({\text{A}\text{l}}_{4}{\text{C}\text{u}}_{9}\). Therefore, the brittleness of the fracture can originate from either the intermetallic character or from insignificant mixing of aluminium and copper. SEM image of the fracture surface of the hybrid (Ag and Zn) interlayer-incorporated Al-Cu FSWed joint \({ \text{J}}_{4}\) is displayed in Fig. 23(d). For joint \({ \text{J}}_{4},\) the intermetallics formed are\(\text{C}\text{u}{\text{Z}\text{n}}_{5}\)&\({\text{A}\text{g}}_{2}\text{A}\text{l}\), which are less brittle and much ductile compared with intermetallics formed in other joints such as\({\text{A}\text{l}}_{2}\text{C}\text{u}\), \({\text{A}\text{l}}_{4}{\text{C}\text{u}}_{9}\), \({\text{A}\text{l}}_{4.2}{\text{C}\text{u}}_{3.2}{\text{Z}\text{n}}_{0.7}\). Combinations of small and some larger-sized dimples on the fracture surface illustrate ductile features. The formation of deep dimples may be due to void expansion in the sample something that Khan et al. also observed, and it suggests that a larger amount of energy was involved in the process that led up to the fracture [42].
The microstructure and mechanical properties of FSWed Al-Cu joints without and with interlayers (Ag, Zn) individually and hybrid mode is studied. The study confirms the formation of complex detrimental intermetallic when Al-Cu is welded without interlayer. In the nugget zone/stir zone incorporation of interlayers have avoided the formation of complex detrimental IMCs and resulted in formation of comparatively soft and ductile IMCs. FSWed Al-Cu joints with hybrid Ag-Zn interlayer has resulted in the formation of IMCs \({\text{A}\text{g}}_{2}\text{A}\text{l}\) &\(\text{C}\text{u}{\text{Z}\text{n}}_{5}\) which is less detrimental compared with joints without interlayer (\({\text{A}\text{l}}_{2}\text{C}\text{u}\), \({\text{A}\text{l}}_{4}{\text{C}\text{u}}_{9}\)). Whereas the intermetallics formed in other joint with Zn as interlayer are\({\text{A}\text{l}}_{4.2}{\text{C}\text{u}}_{3.2}{\text{Z}\text{n}}_{0.7}\), \(\text{C}\text{u}{\text{Z}\text{n}}_{5}\) and for joint with Ag as interlayer are \({\text{A}\text{g}}_{3}\text{A}\text{l}\)or\({\text{A}\text{g}}_{2}\text{A}\text{l}\).
The FSWed Al-Cu joint with hybrid interlayer (Ag and Zn) achieved the highest tensile strength, with a value of 121.78 MPa (i.e., 125% of the Al). This is due to the formation and uniform distribution of Zn-generated thin intermetallic and the distribution of Ag-generated ductile \({\text{A}\text{g}}_{2}\text{A}\text{l}\) intermetallic. The best tensile strength was obtained due to the formation of composite-like structures\({\text{A}\text{g}}_{2}\text{A}\text{l}\),\(\text{C}\text{u}{\text{Z}\text{n}}_{5},\) and a potential intercalated compound of Ag and Zn with Al-Cu in the nugget zone. Tensile fracture surface analysis supports the reason for achieving high tensile strength. For joints with hybrid interlayer where the combination of small and large dimples were noted which confirm the ductile mode of fracture compared with other joints.
Funding
“The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.”
Competing Interests
“The authors have no relevant financial or non-financial interests to disclose.”
Author Contributions
“All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Viresh Payak, and P. Jawahar. The first draft of the manuscript was written by Viresh Payak and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.”
- Venkateswarlu, D., M. Cheepu, and M. Mahapatra. Analysing the friction stir welded joints of AA2219 Al-Cu alloy in different heat-treated-state. in IOP Conference Series: Materials Science and Engineering. 2018. IOP Publishing.
- Mokabberi, S., M. Movahedi, and A. Kokabi, Effect of interlayers on softening of aluminum friction stir welds. Materials Science and Engineering: A, 2018. 727: p. 1-10.
- Aonuma, M. and K. Nakata, Effect of alloying elements on interface microstructure of Mg–Al–Zn magnesium alloys and titanium joint by friction stir welding. Materials Science and Engineering: B, 2009. 161(1-3): p. 46-49.
- Mishra, R.S. and Z. Ma, Friction stir welding and processing. Materials science and engineering: R: reports, 2005. 50(1-2): p. 1-78.
- Sahu, P.K., S. Pal, and S.K. Pal, Al/Cu dissimilar friction stir welding with Ni, Ti, and Zn foil as the interlayer for flow control, enhancing mechanical and metallurgical properties. Metallurgical and Materials Transactions A, 2017. 48(7): p. 3300-3317.
- Tavassolimanesh, A. and A.A. Nia, A new approach for manufacturing copper-clad aluminum bimetallic tubes by friction stir welding (FSW). Journal of Manufacturing Processes, 2017. 30: p. 374-384.
- Pandya, S.N. and J.V. Menghani, Developments of mathematical models for prediction of tensile properties of dissimilar AA6061-T6 to Cu welds prepared by friction stir welding process using Zn interlayer. Sādhanā, 2018. 43(10): p. 1-18.
- Chen, S., et al., Effect of a zinc interlayer on a Cu/Al lap joint in ultrasound-assisted friction stir welding. Journal of Materials Engineering and Performance, 2019. 28(8): p. 5245-5254.
- Wei, Y., et al., Study on microstructure and performance of transient liquid phase bonding of Cu/Al with Al-based interlayers. Vacuum, 2018. 154: p. 18-24.
- Pandya, S.N. and J. Menghani, A preliminary investigation on microstructure and mechanical properties of dissimilar Al to Cu friction stir welds prepared using silver interlayer. Metallurgical and Materials Engineering, 2018. 24(1): p. 45-57.
- Ouyang, J., E. Yarrapareddy, and R. Kovacevic, Microstructural evolution in the friction stir welded 6061 aluminum alloy (T6-temper condition) to copper. Journal of materials processing technology, 2006. 172(1): p. 110-122.
- Mishra, R.S., et al., Friction stir welding and processing VI. 2011: John Wiley & Sons.
- Sorensen, C.D. Innovative technology applications in FSW of high softening temperature materials. in Proc. 5th Int. FSW Symp., Metz, France, September 14-16, 2004. 2004.
- Reynolds, A. and F. Duvall, Digital image correlation for determination of weld and base metal constitutive behavior. Welding Journal-New York-, 1999. 78: p. 355-s.
- Nelson, T., B. Hunsaker, and D. Field. Local texture characterization of friction stir welds in 1100 aluminum. in Proc. 1st Int. Conf. on ‘Friction stir welding’, Thousand Oaks, CA, USA. 1999.
- Colegrove, P. and H. Shercliff, Development of Trivex friction stir welding tool Part 2–three-dimensional flow modelling. Science and technology of welding and joining, 2004. 9(4): p. 352-361.
- Colegrove, P. and H. Shercliff, Experimental and numerical analysis of aluminium alloy 7075-T7351 friction stir welds. Science and Technology of Welding and Joining, 2003. 8(5): p. 360-368.
- Zhang, Y., et al., Review of tools for friction stir welding and processing. Canadian Metallurgical Quarterly, 2012. 51(3): p. 250-261.
- Fonda, R., J. Bingert, and K. Colligan, Texture and grain evolutions in a 2195 friction stire weld. 2004, Los Alamos National Lab.(LANL), Los Alamos, NM (United States).
- Shankar, S., et al., Joint strength evaluation of friction stir welded Al-Cu dissimilar alloys. Measurement, 2019. 146: p. 892-902.
- Hou, W., et al., The role of tool offset on the microstructure and mechanical properties of Al/Cu friction stir welded joints. Journal of Alloys and Compounds, 2020. 825: p. 154045.
- Muhammad, N.A., C. Wu, and W. Tian, Effect of ultrasonic vibration on the intermetallic compound layer formation in Al/Cu friction stir weld joints. Journal of Alloys and Compounds, 2019. 785: p. 512-522.
- Wei, Y., et al., Investigation of interdiffusion and intermetallic compounds in Al–Cu joint produced by continuous drive friction welding. Engineering Science and Technology, an International Journal, 2016. 19(1): p. 90-95.
- Tan, C., et al., Microstructural evolution and mechanical properties of dissimilar Al–Cu joints produced by friction stir welding. Materials & Design, 2013. 51: p. 466-473.
- Fu, S.-W. and C.C. Lee, A study on intermetallic compound formation in Ag–Al system and evaluation of its mechanical properties by micro-indentation. Journal of Materials Science: Materials in Electronics, 2018. 29(5): p. 3985-3991.
- Fedorov, V., et al., Microstructural Study of Al-Ag-Cu-Si Filler Metal for Brazing High-Strength Aluminum Alloys to Stainless Steel. Metals, 2020. 10(12): p. 1563.
- Xue, P., et al., Enhanced mechanical properties of friction stir welded dissimilar Al–Cu joint by intermetallic compounds. Materials science and engineering: A, 2010. 527(21-22): p. 5723-5727.
- Avettand-Fènoël, M.-N., et al., Upon the effect of Zn during friction stir welding of aluminum-copper and aluminum-brass systems. Journal of Manufacturing Processes, 2020. 58: p. 259-278.
- Boucherit, A., et al., Weldability, microstructure, and residual stress in Al/Cu and Cu/Al friction stir spot weld joints with Zn interlayer. The International Journal of Advanced Manufacturing Technology, 2020. 111(5): p. 1553-1569.
- Muhammad, N.A. and C. Wu, Ultrasonic vibration assisted friction stir welding of aluminium alloy and pure copper. Journal of Manufacturing Processes, 2019. 39: p. 114-127.
- Celik, S. and R. Cakir, Effect of friction stir welding parameters on the mechanical and microstructure properties of the Al-Cu butt joint. Metals, 2016. 6(6): p. 133.
- Muthu, M.F.X. and V. Jayabalan, Tool travel speed effects on the microstructure of friction stir welded aluminum–copper joints. Journal of Materials Processing Technology, 2015. 217: p. 105-113.
- Venkateswaran, P. and A.P. Reynolds, Factors affecting the properties of Friction Stir Welds between aluminum and magnesium alloys. Materials Science and Engineering: A, 2012. 545: p. 26-37.
- Kar, A., S. Suwas, and S.V. Kailas, Significance of tool offset and copper interlayer during friction stir welding of aluminum to titanium. The International Journal of Advanced Manufacturing Technology, 2019. 100(1): p. 435-443.
- Al-Roubaiy, A.O., S.M. Nabat, and A.D. Batako, Experimental and theoretical analysis of friction stir welding of Al–Cu joints. The International Journal of Advanced Manufacturing Technology, 2014. 71(9): p. 1631-1642.
- Hsu, C., et al., Al–Al3Ti nanocomposites produced in situ by friction stir processing. Acta Materialia, 2006. 54(19): p. 5241-5249.
- Argesi, F.B., A. Shamsipur, and S.E. Mirsalehi, Preparation of bimetallic nano-composite by dissimilar friction stir welding of copper to aluminum alloy. Transactions of Nonferrous Metals Society of China, 2021. 31(5): p. 1363-1380.
- Muhammad, N.A. and C. Wu, Evaluation of capabilities of ultrasonic vibration on the surface, electrical and mechanical behaviours of aluminium to copper dissimilar friction stir welds. International Journal of Mechanical Sciences, 2020. 183: p. 105784.
- Lin, Y., et al., Influences of Friction Stir Welding and Post‐Weld Heat Treatment on Al–Cu–Li Alloy. Advanced Engineering Materials, 2018. 20(2): p. 1700652.
- Wei, Y., et al., Microstructure and conductivity of the Al-Cu joint processed by friction stir welding. Advances in Materials Science and Engineering, 2020. 2020.
- Swarnkar, R., A. Das, and S.B. Prasad, Enhancement of tensile and microhardness of AA 6082-T6 Friction stir welds via brass interlayers. Russian Journal of Non-Ferrous Metals, 2021. 62(3): p. 333-340.
- Khan, N.Z., A.N. Siddiquee, and Z.A. Khan, Proposing a new relation for selecting tool pin length in friction stir welding process. Measurement, 2018. 129: p. 112-118.